1. Field of the Invention
The present invention relates to elastic wave filter devices having ladder circuit configurations, and more specifically, elastic wave filter devices in which elastic wave filter chips are mounted on boards.
2. Description of the Related Art
Conventionally, elastic wave filters having ladder circuit configurations have been widely used as transmission filters of duplexers in cellular phones. For example, the following Japanese Unexamined Patent Application Publication No. 2010-109694 discloses a duplexer including a transmission filter composed of an elastic wave filter having a ladder circuit configuration and a reception filter composed of a longitudinally coupled resonator-type elastic wave filter.
In the transmission filter of duplexer described in the Japanese Unexamined Patent Application Publication No. 2010-109694, a plurality of series arm resonators is formed in a series arm. A bridging inductance is connected in parallel to one of the series arm resonators on a transmitter terminal side. Further, in parallel arms connecting the series arm and a ground potential, a parallel arm inductance is connected in between a parallel arm resonator and the ground potential. In practice, an elastic wave filter chip with the ladder circuit configuration is mounted on a board. In the elastic wave filter chip, a plurality of series arm resonators and a plurality of parallel arm resonators are connected so as to form the ladder circuit configuration.
Further, the bridging inductance and the parallel arm inductance described above are formed on the board.
In the configuration described in the Japanese Unexamined Patent Application Publication No. 2010-109694, there is a problem that attenuation at a stopband near a passband of the elastic wave filter, namely, a stopband near a transmission band is not large enough. Further, in the duplexer, isolation at a passband, namely, a reception band of the reception filter is not sufficient.